One essential tool for summer cocktails is the muddler. Spring and summer cocktails (as you've seen so far) call for lots of herbs and fruit, which have to be mashed and bruised to release their flavor. A muddler is often called for in recipes for mojitos and mint juleps, and it's nothing more than a heavy round stick with a flat bottom.
You don't have to use a special tool, of course; we often use the back of a spoon or a long pestle. But if you're in the market for a good muddler, here are several that we like.